Deliberate mistake: When Douglas' fingers are chopped off, they are hit from the right side by the blade, but they inexplicably fly directly forward (instead of to the left as they shoot), somersaulting smoothly through the air in a clean, uniform manner. Obviously done for the sake of the 3D effects in the film as they fly directly towards camera, but completely defying the laws of physics.
Deliberate mistake: Empty magazines aren't that heavy, but when Alice kicks it into the face of one of the zombies, it looks like he's being hit with a baseball bat. (00:22:45)
Deliberate mistake: Nearly every night scene inside the house is lit by very bright lights shining in the windows from outside. Yet every time anyone is outside the house, it's still just as dark. There is no light source from which these lights could possibly originate.